Technical Field
The present invention relates to graphite articles, and a process for preparing the graphite articles. More particularly, the invention concerns articles such as graphite electrodes.
Background Art
Graphite electrodes are used in the steel industry to melt the metals and other ingredients used to form steel in electrothermal furnaces. The heat needed to melt metals is generated by passing current through a plurality of electrodes, usually three, and forming an arc between the electrodes and the metal. Electrical currents in excess of 50,000 amperes are often used. The resulting high temperature melts the metals and other ingredients. Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace.
Generally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections, with at least one end of the electrodes comprising female threaded sections capable of mating with the male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes and the pin there between, is referred to in the art as a pin joint.
Given the extreme thermal stress that the electrode and the joint (and indeed the electrode column as a whole) undergoes, mechanical/thermal factors such as strength, thermal expansion, and crack resistance must be carefully balanced to avoid damage or destruction of the electrode column or individual electrodes. For instance, longitudinal (i.e., along the length of the electrode/electrode column) thermal expansion of the electrodes, especially at a rate different than that of the pin, can force the joint apart, reducing effectiveness of the electrode column in conducting the electrical current. Typically, the across grain coefficient of thermal expansion (CTE) of the pin is higher than the across the grain CTE of the electrode. Therefore transverse (i.e., across the diameter of the electrode/electrode column) thermal expansion of the pin being somewhat greater than that of the electrode may be used to form a firm connection between pin and electrode; however, if the transverse thermal expansion of the pin greatly exceeds that of the electrode, damage to the electrode or separation of the joint may result. Again, this can result in reduced effectiveness of the electrode column or even destruction of the column if the damage is so severe that the electrode column fails at the joint section.
As a consequence of the above, the pin joint is a point of concern in an electrode column. To improve the reliability of pin joints, pins are often made from graphite of higher density and strength than the electrode itself. However, increasing the strength and density of graphite pins also increases the manufacturing time and cost of the pin, and hence the cost of the electrode column formed using pin joints. There have been other efforts to improve the reliability of the pin joint. For example, an electrode pin joint may include a reservoir to hold a quantity of pitch binder as a curable binder. While on the furnace, the pitch will reach its softening point and will flow between the threads. Upon more intense heating, the pitch will carbonize in between the threads and hold the adjacent threads together. Variations on this concept include the pin having one or more flow channels and/or the pin joint including more than one pitch reservoir or the location of the reservoir being varied.
In the past, efforts have also been taken to eliminate the pin from the joint in order to improve the performance of the electrode column system. Prior attempts to eliminate the pin, which have been attempted, include a threaded electrode end or other electrode mating means being employed. For example electrodes have been made which include an integral threaded tang at one end of the electrode, also known as a pinless joint. Industry acceptance of a pinless joint has lagged, however, since the strength of the graphite in the electrode is viewed by some as not sufficient to maintain the integrity of the electrode column. For the above reasons and others, the joint between two adjacent electrodes in an electrode column is an area of concern for an operator of an electric arc furnace.
A Soderberg Paste electrode is an example of a prior attempt to produce a pinless electrode. The Soderberg electrode is a continuously formed electrode used in an electric arc furnace, in which a mixture of petroleum coke and coal-tar pitch is continuously added to a steel casing and is baked as it passes through the heated casing, such that the baked electrode emerging into the furnace continuously replaces the electrode being consumed. Since these electrodes are baked and not graphitized, their performance is not suitable for use in electric arc steelmaking. The paste electrodes are typically used in arc furnaces for manufacturing ferroalloys, aluminum, nickel, copper and other non-ferrous applications.
In light of the above issues, electrode joint designs have been standardized over the years. These standards specify the height and diameter designs for pins along with the parameters for the threads of the socket of an electrode. In addition to standards regarding the electrode joint, standards have also been drafted and approved regarding the length and diameter of the electrode. Examples of one such standard are IEC 60239 and JIS R7201. In each one of these standards the length of the electrode varies from no more than 2900 mm to about 825 mm and the diameter of the electrode may vary from between 765 mm to 352 mm for an electrode of 2900 mm to 2275 mm in length.
Another issue for a steel manufacturer is downtime and other problems associated with electrode additions to the arc furnace. Each time another electrode is to be added to an electrode column or a new column is to be added to the furnace, the furnace must be shut down while the electrode or electrode column is added. Typically, for a furnace where three electrode columns are in simultaneous operation, the equivalent of one electrode will be consumed over the course of about one eight (8) hour shift. Thus, to add an electrode to a column, or to exchange a shortened column with one of longer length, the furnace must be shut down about three times during every twenty-four (24) hour period.
An example of how electrode columns are installed on a furnace is illustrated in FIGS. 3 and 4. FIG. 3 is a top view of the electric arc furnace depicted in FIG. 4. As illustrated, the three electrode columns 104, 120, and 130 are installed in furnace 102. Typically a furnace operated on alternating electric current will have three such columns, where a furnace operating on direct electrical current will use larger diameter electrodes in a single electrode column.
When a particular electrode column is consumed, typically the electrical current to create the arc to reclaim the steel is turned off and the remainder of the consumed column is removed from the furnace. The power is then turned on and the current is transmitted through one or more of the remaining electrode columns and/or replacement column. Depicted in FIG. 4 is a view of electrical arc furnace 102 which shows two (2) electrode columns 104 and 120. Column 104 includes three (3) electrodes 106, 108, and 110. The joints between the electrodes of column 104 are represented as reference numerals 112 and 114.
Electrode column 120 includes two electrodes 122 and 124. In the depicted example, an electrode, such as electrode 110 may be added to electrode column 104 by the use of an electrode robot 126. As shown robot 126 is used to add a third electrode to a column already comprising more than one electrode. Robot 126 may be used to align and rotate the electrode being added to the column to engage a threaded portion of the top joint element of the electrode directly below the electrode being added. Robot 126 may travel along rails 128, shown in FIG. 4 or may be positioned over the column by the use of an overhead crane.
Similar to what was previously discussed, when an electrode is being added to a column, the electrical current being passed through a column of the electrodes in furnace 102 is turned off and the significant production time is lost due to this change.
One method of reducing electrode additions at the furnace is to join two relatively shorter electrodes together prior to delivery to the steelmaker, as described in published U.S. patent application 2006/0140244. However, this approach has the disadvantage that each of the shorter electrodes must be machined to have its own threaded tang and socket portions prior to assembly, requiring the machining of four threaded sections instead of two for a single electrode. The need to machine four threaded sections requires additional labor and time, and wastes the high value graphite material that is machined away to make the threaded section. Thus, there is a need for a monolithic electrode, that is, an electrode without an added joint that can also provide the user with a longer period of productivity between electrode additions.